1. Field of the Invention
The present invention relates to a technique for improving the flatness of an electrode surface in an inner surface of a substrate to enhance contrast in an in-plane switching liquid crystal display device, and particularly relates to a technique for using a coating material to form an entire capacitance-holding electrode structure for holding an electric field that is applied to the liquid crystal, whereby a level of cleanliness is maintained in a manufacturing step, a backlight transmissivity is assured to improve brightness, and a lower-cost panel is achieved by reducing material costs.
2. Description of the Related Art
An in-plane switching mode (also referred to as the IPS method) liquid crystal display device features a wide viewing angle. A transmissive liquid crystal display device is used in personal computers, televisions, and various other devices having mainly an indoor application, and a reflective or semi-transmissive liquid crystal display device is mainly used in mobile phones and other portable information terminals that are used in various environments including outdoors in clear weather as well as in a dark room. The transmissive liquid crystal display device controls luminous energy by modifying an array direction of the liquid crystal molecules when light from a backlight provided to the far side of the substrate on the side opposite from the viewing side is allowed to pass through the substrate. On the other hand, the reflective liquid crystal display device has a reflective film on the substrate on the side opposite of the viewing side. The reflective film is ordinarily used in combination with a pixel electrode or an opposing electrode, and an image is displayed by providing very small concavities and convexities to the reflective surface and scattering and reflecting external light that enters from the viewing side. The semi-transmissive liquid crystal display device is provided with a transmission part (transmission display part) and a reflection part (reflection display part), whereby both transmissive and reflective functions are provided, and display is made possible in various environments.
In a liquid crystal display device, including transmissive-, reflective-, and semi-transmissive-type devices, the contrast characteristics are one condition for high-quality display. One cause of a reduction in contrast in a liquid crystal display device is misalignment of the liquid crystal molecules caused by concavities and convexities of the electrode on the main surface (inner surface) of the substrate in contact with the liquid crystal layer. The orientation film is formed on the topmost surface of the main surface of the substrate in contact with the liquid crystal layer. Liquid crystal alignment controllability is imparted to the orientation film and initial alignment of the liquid crystal molecules in contact therewith is defined. Glass, plastic, or another insulating substrate is used as the substrate.
Electrodes (ordinarily, pixel electrodes and common electrodes; common electrodes are also referred to as opposing electrodes) for applying an electric field to liquid crystal in the main surface of the substrate constituting the liquid crystal display device, drive elements (ordinarily, thin film transistors) for driving these electrodes, and various wiring and electrodes are mutually formed in a layered state together with an insulating film and a protective film. The pixel electrodes and opposing electrodes are composed of a transparent electroconductive film typified by ITO or the like formed by sputtering. In an IPS-type liquid crystal display device, pixel electrodes, opposing electrodes, drive elements, wiring, and electrodes are formed only on one of the substrate sides (First substrate: TFT substrate). A light-blocking film (black matrix: BM; color filter: CF) is formed on another substrate (Second substrate: opposing substrate or CF substrate) facing the TFT substrate. In a vertical electric field-type (TN-type), an opposing electrode is formed on the CF substrate side. There are also configurations in which an electrode is provided to the CF substrate side even in a VA-type or another type of liquid crystal display device without limitation to a TN-type.
The substrate inner surface described above, i.e., the main surface of a TFT substrate that forms the thin film transistor has considerable concavities and convexities due to the complex cross-sectional shape of the thin film transistor as such. Accordingly, the surface of the electrode formed on the upper layer of the TFT substrate also has concavities and convexities that reflect the structure of the underlying layer. The concavities and convexities of such an electrode disrupt the distribution of the electric field that acts on the liquid crystal molecules, degrades blackness, and causes a reduction in the contrast. In an IPS device, the concavities and convexities of such an electrode contribute considerably to a reduction in contrast because liquid crystal molecules are rotated in a direction parallel to the substrate surface. Japanese Laid-open Patent Application No. 2000-111935 discloses an attempt to dispose a flattened layer on an electrode in order to flatten the concavities and convexities of the substrate inner surface, wherein a resin layer is inserted between the electrode and the insulating layer in a vertical electric field-type liquid crystal display device and the layer is flattened.
The electroconductive resin film in Japanese Laid-open Patent Application No. 2000-111935 is disclosed as being formed between the pixel electrode and the orientation film in the TFT substrate (paragraph [0038] in the specification of Japanese Laid-open Patent Application No. 2000-111935). A structure is disclosed in which the electroconductive resin film increases maximum brightness by reducing the thickness of the orientation film when liquid crystal having spontaneous polarization is used, and improves the contrast as a result (paragraph [0041] of the specification of Japanese Laid-open Patent Application No. 2000-111935).
In other words, it is not the intension of Japanese Laid-open Patent Application No. 2000-111935 to improve the contrast by flattening the concavities and convexities of the electrode of the main surface of the substrate. Additionally, the problem in which the IPS-type liquid crystal display device considerably affects the electric field that acts on the liquid crystal molecules due to the concavities and convexities of the electrode of the main surface of the substrate as well as the method for solving the issue are not addressed. The effect of the concavities and convexities of the electrode on the main surface of the substrate in an IPS-type liquid crystal display device on the contrast of the display is very high in comparison with a TN-type device. An improvement in the contrast in an IPS-type liquid crystal display device cannot be highly anticipated even if a configuration is adopted in which an electroconductive resin film such as that described in Japanese Laid-open Patent Application No. 2000-111935 is merely overlaid on the pixel electrode and the opposing electrode.
In the case that a coated transparent conductive electrode is inserted between the electrode and the capacitance-holding insulating film for holding an electric field applied to the liquid crystal and the electrode plane is flattened, the contrast can be improved by flattening without compromising the electroconductivity of the electrode.
In order to insert such a coated transparent electroconductive film between the electrode and the insulating film, however, ITO or another metal must be deposited as a film by sputtering or another vacuum deposition method, the transparent electroconductive film must ordinarily be formed thereafter in a coating step (nozzle discharge, spin coating, inkjet, and other methods) in an environment in contact with the atmosphere, after which chemical vapor growth or another film formation step in a vacuum device must again be used in order to form an insulating film. The coated transparent electroconductive film material includes metal microparticles, organic polymer, or the like, and impurities in the atmosphere become deposited on the surface in the coating step. Consequently, residue and impurities are diffused in the layered films when processing is carried out in a vacuum and high-temperature condition, and the operation of the semiconductor may be degraded or the manufacturing device may become contaminated.
There is also a problem in that flattening is insufficient when using a coated film alone. The transmissive display part has thin film transistor-induced concavities and convexities described above and the reflective display part also has scattering reflective electrode-induced concavities and convexities. The thickness of the coated transparent electroconductive film must be considerable in order to sufficiently flatten the concavities and convexities. Simultaneously assuring sufficient transparency with such a film thickness presents a problem in terms of implementation because there are many limitations in the selection or the like of coating materials.
There is a further problem in that ITO and the like that are used for forming a film by sputtering have become rare metals in recent years due to worldwide resource shortages. In the layered structure in which a coated transparent electroconductive film is inserted between the capacitance-holding insulating film and the electrode, there is a problem in that the structure is not suited to reducing costs overall because the amount of ITO and other rare metals used in a pixel electrode or a common electrode is not different from a conventional structure that does not use a coated electroconductive film, and has the additional cost of the material for coating.